powerpc/mm: Fix CONFIG_PPC_KUAP_DEBUG on PPC32

CONFIG_PPC_KUAP_DEBUG is not selectable because it depends on PPC_32
which doesn't exists.

Fixing it leads to a deadlock due to a vital register getting
clobbered in _switch().

Change dependency to PPC32 and use r0 instead of r4 in _switch()

Fixes: e2fb9f54 ("powerpc/32: Prepare for Kernel Userspace Access Protection")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/540242f7d4573f7cdf1b3bf46bb35f743b2cd68f.1587124651.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/entry_32.S

@@ -732,7 +732,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_SPE)
 	stw	r10,_CCR(r1)
 	stw	r1,KSP(r3)	/* Set old stack pointer */
 
-	kuap_check r2, r4
+	kuap_check r2, r0
 #ifdef CONFIG_SMP
 	/* We need a sync somewhere here to make sure that if the
 	 * previous task gets rescheduled on another CPU, it sees all

arch/powerpc/platforms/Kconfig.cputype

@@ -397,7 +397,7 @@ config PPC_KUAP
 
 config PPC_KUAP_DEBUG
 	bool "Extra debugging for Kernel Userspace Access Protection"
-	depends on PPC_KUAP && (PPC_RADIX_MMU || PPC_32)
+	depends on PPC_KUAP && (PPC_RADIX_MMU || PPC32)
 	help
 	  Add extra debugging for Kernel Userspace Access Protection (KUAP)
 	  If you're unsure, say N.